So, I'm a big fan of this HDPE Panel you can get at Menards. It is about 55$ for 4' x 8' x 1/4" sheet. Unlike a epoxy or liquid rubber plywood aquarium you don't have to paint it on, or deal with an epoxy smell as it cures. It is also a great liner for the aquarium that can flex if needed without cracking. The problem is finding a fish safe sealant to seal the joints if you want to use it in a plywood aquarium build. I'm going to show you the adhesion properties of three different caulks that may be good candidates. I'll give you my two scents on the safeness of the caulking, but you need to do your own research on this topic.
The three candidates I tried were Butyl Rubber, 100% Silicone Sealant for Plastic, and 100% Silicone,.
I ran a bead down the side of two HDPE Panels and let the caulking cure. After curing, I put a weight on the panel to see how much weight it would hold. Not very scientific, but it gets the job done.
First up 100% silicone.
I was actually pretty surprise how much weight this caulking could hold. I've been told that silicone will not stick at all, yet it could hold the weight of a utility knife. I tried putting a tape measure on it and it failed. In this case, I believe it held better than what I thought it would do because of the rough texture.
Here is the silicone that has taken the imprint of the roughness of the HDPE Panel which creates more surface area to stick too and prevents side movement.
Now for the Butyl Rubber.
This caulking by far had the best adhesion properties of all three, but it takes a very long time for it to cure. I put the bead down the side and came back 24 hours later and the outside was hardened, but the interior was still soft. So the side would just bend when I put a weight on it. I let it cure for 3 more days, and it became more rigid. Here it is holding the weight of at 16' tape measurer. I also put a 25' tape measure on it, which it held for a while than started to bend. From my experience, this stuff takes a long time to cure which may be a fishsafe problem.
Finally my favorite, 100% Silicone for Plastics.
Here is a picture of the caulking holding a 16' tape measure prepared the same way as the other caulks. It also held the 25' tape measure for a bit, but failed after I shook it a few times.
Basically, I think any of these caulking would function well in a plywood aquarium where the water pressure is pushing the caulking against rigid sides.
My two cents on fish safe component of the caulking. The 100% silicone is try and true in the DIY community. Of course we all recommend using aquarium silicone for those of us that don't want to do our research or take chances. Butyl rubber is safer than PVC piping, however companies sneak other stuff in their product you have to be careful about. Like this white lightening has about 5% titanium dioxide which could cause problems for corals. Also, the solvent doesn't evaporate very fast, which could poison your fish as it leaches into the water. The 100% silicone for plastics has some pretty nasty stuff in it to help it cure to plastics. I think as long as it is fully cured your fish will be ok, and I've read some forums where they used this product without problems. I've decided to use this product in my HDPE Panel build, so I would suggest waiting tell I'm done with my build. I think my fish will be happy.
The three candidates I tried were Butyl Rubber, 100% Silicone Sealant for Plastic, and 100% Silicone,.
I ran a bead down the side of two HDPE Panels and let the caulking cure. After curing, I put a weight on the panel to see how much weight it would hold. Not very scientific, but it gets the job done.
First up 100% silicone.
I was actually pretty surprise how much weight this caulking could hold. I've been told that silicone will not stick at all, yet it could hold the weight of a utility knife. I tried putting a tape measure on it and it failed. In this case, I believe it held better than what I thought it would do because of the rough texture.
Here is the silicone that has taken the imprint of the roughness of the HDPE Panel which creates more surface area to stick too and prevents side movement.
Now for the Butyl Rubber.
This caulking by far had the best adhesion properties of all three, but it takes a very long time for it to cure. I put the bead down the side and came back 24 hours later and the outside was hardened, but the interior was still soft. So the side would just bend when I put a weight on it. I let it cure for 3 more days, and it became more rigid. Here it is holding the weight of at 16' tape measurer. I also put a 25' tape measure on it, which it held for a while than started to bend. From my experience, this stuff takes a long time to cure which may be a fishsafe problem.
Finally my favorite, 100% Silicone for Plastics.
Here is a picture of the caulking holding a 16' tape measure prepared the same way as the other caulks. It also held the 25' tape measure for a bit, but failed after I shook it a few times.
Basically, I think any of these caulking would function well in a plywood aquarium where the water pressure is pushing the caulking against rigid sides.
My two cents on fish safe component of the caulking. The 100% silicone is try and true in the DIY community. Of course we all recommend using aquarium silicone for those of us that don't want to do our research or take chances. Butyl rubber is safer than PVC piping, however companies sneak other stuff in their product you have to be careful about. Like this white lightening has about 5% titanium dioxide which could cause problems for corals. Also, the solvent doesn't evaporate very fast, which could poison your fish as it leaches into the water. The 100% silicone for plastics has some pretty nasty stuff in it to help it cure to plastics. I think as long as it is fully cured your fish will be ok, and I've read some forums where they used this product without problems. I've decided to use this product in my HDPE Panel build, so I would suggest waiting tell I'm done with my build. I think my fish will be happy.
. Shame that Home Depot or lowes don't care these panels. Cause the nearest menards is about 1.5 hours from me 
